206 



GENETICS 



ject to regarding the male factor as nothing positive, 

 but simply the absence of femaleness, by assuming 

 that a universal factor of maleness (ra) is present in 

 all cases, as shown in Figure 61. 



Thus in type I of this scheme it is only when the 

 dominant female factor F is entirely absent that male- 

 ness becomes expressed in the somatoplasm, while in 



type II it is neces- 

 sary to have a double 

 dose of the factor F 

 in order to produce 

 a female, since a 

 single dose results in 

 a male. 



All of these three 

 theoretical schemes 



FIG. 61. Diagram to show the neo-Men- 

 delian theory of the heredity of sex with 

 Morgan's modification, making male- 

 ness (m) present as a positive character 

 in every gamete. 



agree in assuming 

 that one sex is hetero- 

 zygous, while the other 

 is homozygous and that femaleness is the result of 

 an added factor in excess of maleness. 



The evidence for these conclusions has been ob- 

 tained chiefly from four sources : first, from a mi- 

 croscopical examination of the germ-cells ; second, 

 from castration and regeneration experiments ; third, 

 from the results of. hybridization in "sex-limited 

 inheritance"; and fourth, from the behavior of 

 hermaphrodites in heredity. 



